You've hit on one of the (many) fundamental differences between TiVo-software on a V6 & Horizon-software on a TV360.
V6 are fundamentally recording-centric, with each box being totally independent with full inter-box streaming. Meaning separate & independent recording pools that are share-able/
TV360 is an online-centric OS, which is designed around one master-box with the HDD, and additional boxes all being TV360-mini without recording capability. Thus all boxes access the one collective recording pool.
Converting multiple V6 into TV360-master provides some mitigation and allows each box to retain the HDD-capacity. But the software design around one central recording pool is unavoidable.
No idea on the recording error message, hopefully someone who uses a TV360 may be along to assist. I still use a V6.